1. Field of Invention
This invention relates to a process control system that performs control and operation of a plant by controlling the process variables thereof, such as temperature, pressure, etc.
2. Description of the Prior Art
Conventional process control systems usually comprise an information command station and a plurality of control stations connected to a communication bus, wherein the control stations handle plant control and the command station handles plant monitoring and operation. According to the scale of the plant that is the target of control, a plurality of control stations is distributed and arranged throughout the plant.
FIG. 1 shows a conventional process control system, wherein information command station ICS and a plurality of field control stations FCS are connected to communication bus BS. The field control stations FCS control plant PLT. According to the scale of the plant, a plurality of control stations are distributed and arranged throughout the plant. Information command station ICS comprises display unit 1 comprising, for example, a CRT (cathode ray tube) which has a man-machine interface function, etc.; display control unit 2 which controls the display unit 1 and causes display unit 1 to show various screens or windows required for plant control and monitoring; input unit 3 which may comprise, for example, a keyboard, a mouse, etc, and handles data input for plant operation and control, etc; monitoring unit 4 which monitors the plant; operation unit 5 which carries out operations for plant control with the results of monitoring and operation of the plant being displayed on the display unit 1; and communication unit 6 which exchanges data with each control station via communication bus BS according to a specified communication protocol.
Field control station FCS comprises control unit CU which handles control of plant PLT; and input-output unit (I/O unit) which receives various types of signals as inputs from or transmits signals as outputs to plant PLT. For example, an I/O unit inputs and/or outputs analog signals of 4 to 20 mA or 1 to 5 V, thermocouple signals, contact signals, etc. The FCS further comprises communication unit 11 which exchanges data with the information command station or with each field control station via the communication bus BS according to a specified protocol.
In each plant PLT, sensor SN detects a process value, such as temperature and pressure. The opening of valves V1 and V2 is control led by the manipulated signal provided by the field control station FCS. For example, an analog signal of 4 to 20 mA or 1 to 5 V provided by sensor SN is applied to I/O unit 10. Based on this input, control unit CU carries out control calculations and determines a manipulated variable. This manipulated variable is outputted from I/O unit as an analog signal of 4 to 20 mA or 1 to 5 V, which is used to control the opening of valves V1 and V2.
In the system of FIG. 1, plant control is executed by controlling the process variables, such as temperature and pressure, that exist in the plant.
FIG. 2 shows the status of storing data nd programs of a function block process in the control units of the system. Function blocks are the elements of a control system and, for example, include PID computation block (PID), indication block (PVI), etc. Various control systems are constructed by combining these function blocks. As shown in FIG. 2, data and programs are stored in database DB and program library PL, respectively. Data blocks to which tag names are attached are stored in the database DB. Examples of these tag names are: FIC101, PIC102, PI201. Programs used for processing the data of the function blocks corresponding to each data block are prepared in program library PL. More than one program is prepared because there are two or more algorithms for the processing of the function blocks.
Interpreter INT reads the data blocks in turn, selects and calls up a processing program specified in a read data block from the program library PL. The called up processing program executes calculation process using the data in the corresponding data block. For example, interpreter INT reads data block fIC101 , selects and calls up a processing program PID specified in the data block from program library PL. The processing program PID executes calculation processing using the data in the data block FIC101. However, the conventional system of FIG. 1 has the following problems:
Since each function, such as monitoring and control, is designed to have its optimum function, it is necessary to have individual dedicated hardware and software board (OS) for each function. For example, the control function mounted in control unit CU is designed on the premise of a dedicated interface for the I/O unit 10. Accordingly, physical restrictions are imposed on the control function so that the control function and the I/O unit 10 must be mounted on the same hardware, i.e. field control station fCS. In addition, if a function is to be mounted on a separate device, reconfiguration of the function is required, such as program modification or recompiling.
As shown in FIG. 2, programs and data are maintained separately. For the programs, algorithms for processing all types of function blocks in the system are integrated into the library. Thus, when changing an algorithm for function blocks, the entire program library must be updated even if only one algorithm for one type of function block is changed. Moreover, to change or add a function, it is necessary to recompile and reload the entire program library, which requires temporrary shut down of the entire system.
As shown in the program library of FIG. 2, the functions not being used are mounted and integrated together with the functions actually used. For this reason, the required range of shared parts increases regardless of whether the system is large or small in scale. This results in increased system cost even when constructing a small scale system. In other words, it is not possible to group functions according to the user""s needs.
Accordingly, an object of the invention is to overcome the afore-mentioned and other problems, disadvantages and deficiencies of the prior art.
Another object is to provide a process control system which is more cost effective and offers better scalability or expandability of process control systems.